Fresnel reflectors are useful for both focusing or concentrating light, such as sunlight, and generating collimated light. They have practical applications ranging from heliostat arrays to pocket flashlights. However, Fresnel reflectors are prone to a phenomenon known as facet shading or cosine loss, which occurs when one facet in the reflector occludes the reflected or projected light of another facet in the reflector.
In order to reduce or compensate for facet shading, designs for Fresnel reflectors often increase the height of receivers for reflected light or sources for projected light. Or the designs increase the distance between facets and compensate for the loss of optical efficiency by increasing the area that the reflector system covers. Of course, all of these increases come at a cost, in terms of resources and space.